Dissecting Functional Heterogeneity in the Human CD8⁺ T Cell Compartment to Characterize Superior Effector Responses

De Simone, Gabriele (2020). Dissecting Functional Heterogeneity in the Human CD8⁺ T Cell Compartment to Characterize Superior Effector Responses. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.000124f7

Abstract

The constant development of technologies capable to define features at the level of single cells enabled to investigate cellular heterogeneity that was previously inaccessible due to population averaging. In this work, I took advantage of cutting-edge single cell technologies to unravel functional diversification within the human CD8⁺ T cell compartment, with the final aim to identify T cell subsets that can exert superior effector responses against viruses and cancer. In mice, the capability to exert effector responses correlates with TCR sensitivity for self-Ags, as assessed by indirect measurement of surface expression of CD5. Equivalent functional heterogeneity has not been previously described in humans. We investigated the CD8⁺ naïve T (T_N) cell pool, and discovered the presence of two discrete subsets with distinct effector differentiation potential, as defined by the differential expression of the chemokine receptor CXCR3. Among these, CXCR3⁺ (T_NR3⁺) cells showed increased cytokine production and effector differentiation potential, both in vitro and in vivo. These features correlated with the expression of T cell receptors suggesting enhanced interactions with p:MHC-I complexes. Thus, functional heterogeneity exists in CD8⁺ TN cells, thereby showing that the effector differentiation potential is predetermined at the level of pre-immune repertoire.

Stem-like characteristics of T cells have been a long-standing interest of the lab. T cell memory relies on the generation of antigen-specific precursors with stem-like properties. However, precise definition of these progenitors and how they shape memory and effector differentiation is still unclear. We characterized subsets of clonally, phenotypically, functionally, epigenetically and transcriptionally distinct stem-like CCR7⁺ CD8⁺ memory T cells, identified by the presence or absence of inhibitory receptors PD-1 and TIGIT. We found that PD-1^-TIGIT^- progenitors were committed to a functional lineage, while PD-1⁺TIGIT⁺ progenitors were committed to a dysfunctional, exhausted-like lineage. Collectively, these data demonstrate the existence of parallel differentiation programs within the early-differentiated CD8⁺ memory T cell compartment, providing fundamental information for the identification of T cells that are more suitable for cell-based immunotherapies.